The polyphenylene ethers are known and described in numerous publications, including Hay, U.S. Pat. Nos. 3,306,874 and 3,306,875; and Stamatoff, U.S. Pat. Nos. 3,257,357 and 3,257,358, all incorporated herein by reference. They are useful for many commercial applications requiring high temperature resistance and, because they are thermoplastic, they can be formed into films, fibers and molded articles. In spite of these desirable properties, parts molded from polyphenylene ethers are somewhat brittle due to poor impact strength. In addition, the relatively high melt viscosities and softening points are considered a disadvantage for many uses. Films and fibers can be formed from polyphenylene ethers on a commercial scale using solution techniques, but melt processing is commercially unattractive because of the required high temperatures needed to soften the polymer and the problems associated therewith such as instability and discoloration. Such techniques also require specially designed process equipment to operate at elevated temperatures. Molded articles can be formed by melt processing techniques, but, again, the high temperatures required are undesirable.
It is known in the art that the properties of the polyphenylene ethers can be materially altered by forming compositions with other polymers. For example, U.S. Pat. No. 3,379,792 discloses that flow properties of polyphenylene ethers are improved by preparing a composition thereof with from about 0.1 to 25 parts by weight of a polyamide. In U.S. Pat. No. 3,361,851 polyphenylene ethers are formed into compositions with polyolefins to improve impact strength and resistance to aggressive solvents. In U.S. Pat. No. 3,383,435, incorporated herein by reference, there is provided a means to simultaneously improve the melt processability of the polyphenylene ethers and upgrade many properties of polystyrene resins. The latter patent discloses that polyphenylene ethers and polystyrene resins, including many modified polystyrenes are combinable in all proportions to provide compositions having many properties improved over those of either of the components.
Preferred embodiments of U.S. Pat. No. 3,383,435 are compositions comprising a rubber modified high-impact polystyrene and a poly(2,6-dialkyl-1,4-phenylene)ether. Such compositions are important commercially because they provide both an improvement in the melt processability of the polyphenylene ether and an improvement in the impact resistance of parts molded from the compositions. Furthermore, such compositions of the polyphenylene ether and the rubber modified high-impact polystyrene may be custom formulated to provide pre-determined properties ranging between those of the polystyrene resin and those of the polyphenylene ether by controlling the ratio of the two polymers. The reason for this is that the compositions exhibit a single set of thermodynamic properties rather than the two distinct sets of properties, i.e., one for each of the components of the composition, as is typical with compositions or blends of the prior art.
In preferred embodiments of U.S. Pat. No. 3,383,435, rubber modified polystyrenes are used in compositions with polyphenylene ethers because they provide an increase in toughness, e.g., resistance to impact fracture. However, the use of commercially available graft type rubber modified high impact polystyrenes, such as the LUSTREX HT-88 employed in Example 7 of U.S. Pat. No. 3,383,435, causes a sacrifice in transparency, possibly due to scattering of light by the dispersed elastomeric particulate phase in the rubber modified polystyrene. Moreover, the average such rubber particles are greater in diameter than about 1 micron, because it has been stated often that if smaller particles are used, impact strength will be lowered.
In British Pat. No. 1,180,085, it is disclosed that careful attention to the morphology, i.e., the size and nature of the dispersed phase, in copolymers comprising alkyl acrylates and styrene monomers, rendered impact resistant by inclusion of rubber particles, will lead to enhanced impact strength, without loss in transparency, even though very small particles are used. It is suggested in the British patent that the acrylate copolymer should fill the inside or dispersed globular particles, and that each particle should have a thin rubbery membrane or shell surrounding it.
The said British patent states, however, that to try the same technique with polystyrene alone is unsuccessful--the product will mold into a material of low impact strength and, also significantly, the resin will be poor in transparency. On page 15 of the British patent, this failure of polystyrene to qualify as a suitable, useful composition is laid, at least in part, to the magnitude of the difference between the refractive index of the rubber component, and the resin, i.e., homopolystyrene, component. It is stated that such difference must not be in excess of 0.005 refractive index units.
Thus the British patent expressly teaches that the only useful compositions must always include an alkyl methacrylate in the thermoplastic matrix and further that there never can be greater than a 0.005 difference in refractive index between that of the rubber and that of the thermoplastic. Moreover, there is no suggestion that such compositions will be useful to modify other thermoplastic resins, and carry their transparency into such modified compositions and, particularly, there is no suggestion to use any such resins, and especially entirely polystyrene based resins, in compositions with polyphenylene ethers.
In view of the above, it has now unexpectedly been found that a rubber modified polystyrene in which polystyrene is englobed by a diene rubber membrane and dispersed in tiny particles throughout a polystyrene matrix will combine in all proportions with polyphenylene ether resins to give compositions which are both transparent and tough. The unusual morphology of the rubber modified polystyrene seems to be responsible for the observed advantages. Compositions of polyphenylene ether with a rubber modified polystyrene containing particles in which many polystyrene inclusions are present, are both opaque and not as tough. Moreover, a refractive index difference of 0.05 units has been found to be acceptable in the present compositions, even though the British patent states that the difference should be no more than 0.005 units (in cases where polyphenylene ethers were not present).